A stereoscopic microscope apparatus as an example of a microscope apparatus can stereoscopically observe an object with protrusions and recesses as if the object is viewed by both eyes. Therefore, a distance relationship between a tool, such as tweezers, and an object can be easily recognized in an operation with the microscope. Thus, the microscope apparatus is particularly effective in a field that requires precise procedures, such as precision machinery industry and anatomy or surgery of living organisms. In such a stereoscopic microscope apparatus, an optical system of the luminous flux entering left and right eyes is at least partially separated to cause the optical axes to intersect over the surface of the object to obtain a parallax for stereoscopically observing the object. Enlarged images of the object viewed from different directions are created, and the images are observed through an eyepiece to stereoscopically view a minute object.
In the stereoscopic microscope apparatus, an example of a typical method for obtaining a stereoscopic vision includes a parallel stereoscopic microscope apparatus (parallel single-objective binocular microscope apparatus). As shown in FIG. 30(a), a parallel stereoscopic microscope apparatus 100′ includes one objective lens 1′ and two observation optical systems 2′ for right eye and left eye arranged parallel to the optical axis of the objective lens 1′. Each of the observation optical systems 2′ usually includes a variable power mechanism which will be called a variable power optical system 3′ below. Each of the observation optical systems 2′ also includes an imaging lens 4′.
In the parallel stereoscopic microscope apparatus 100′, the objective lens 1′ that has brought the focus position in line with the surface of the object plays a role of guiding the parallel luminous flux to the following variable power optical systems 3′ for left and right eyes. The parallel luminous flux ejected from the objective lens 1′ is divided into the two variable power optical systems 3′ and is separately delivered to the left and right eyes. As shown in FIG. 30(b), each of the two variable power optical systems 3′ is provided with a diaphragm S′. The position of the entrance pupil here is a position where a diaphragm image formed by a lens group in the variable power optical system 3′ closer to an object O than the diaphragm S′ is created. In the parallel stereoscopic microscope apparatus 100′ with the configuration, the definition of the objective lens numerical aperture is different from that of a normal objective lens numerical aperture as shown in FIG. 30(a). More specifically, if the medium between the object O and the objective lens 1′ is air, the normal objective lens numerical aperture is defined by sine of a half angle α of an angle of aperture of a luminous flux which is spread over the entire aperture of the objective lens 1′ from the light ejected from a point on the optical axis of the object O. The objective lens numerical aperture in the parallel stereoscopic microscope apparatus 100′ is defined by sine of a half angle β of an angle of aperture when the light ejected from a point on the optical axis of the object O is spread to the maximum diaphragm diameter of the diaphragm S′ of one of the variable power optical systems 3′.
FIG. 30(b) is a diagram enlarging the objective lens 1′ and part of the variable power optical system 3′ of one side of FIG. 30(a). The light exited from the center of the surface of the object O enters the objective lens 1′ to form a parallel luminous flux, and the parallel luminous flux enters the variable power optical system 3′. Since the objective lens 1′ sufficiently satisfies the sine conditions, the parallel luminous flux diameter is twice the product of a focal length fobj of the objective lens and the objective lens numerical aperture sin β. The luminous flux needs to be guided to the variable power optical system 3′ to exhibit the performance in accordance with the objective lens numerical aperture. Assuming that the effective diameter of the variable power optical system 3′ is Dep, a relationship of the effective diameter Dep≧the parallel luminous flux diameter (=2·fobj·sin β) needs to be satisfied. In other words, the objective lens numerical aperture sin β in the parallel stereoscopic microscope apparatus 100′ depends on the size of the effective diameter Dep of the variable power optical system 3′. As described, the stereoscopic microscope apparatus includes two optical paths for left eye and right eye for stereoscopic vision, and since the left and right optical paths are adjacent, the enlargement of the effective diameters Dep of the variable power optical systems 3′ is synonymous with the enlargement of the distance between left and right optical axes of the variable power optical systems 3′. To put it plainly, it can be stated that the distance between the left and right optical axes of the variable power optical systems 3′ determines the numerical aperture of the parallel stereoscopic microscope apparatus 100′. The variable power optical systems 3′ are constituted as afocal variable power optical systems in which an entering luminous flux and an ejected luminous flux are parallel, and the imaging lenses 4′ arranged subsequently form an image. The magnification of the afocal variable power optical system (hereinafter, called “afocal magnification”) is calculated by dividing the parallel luminous flux diameter on the incident side by the parallel luminous flux diameter on the ejection side. The magnification of the image can be calculated by dividing a value fzoom, which is obtained by multiplying the focal length of the imaging lens 4′ by the afocal magnification, by the focal length fobj of the objective lens 1′. In recent years, demand for a stereoscopic microscope apparatus capable of observing a wide variable power range by one apparatus is increasing along with the diversification of applications. Consequently, a variable power optical system is proposed in which the variable power range is enlarged while the total length is controlled (for example, see Patent Literature 1).